1. Field of the Invention
The present invention relates to image forming apparatus for forming an image by developing an electrostatic latent image formed on a latent image carrier. More particularly, the present invention relates to an image forming apparatus designed to reduce the occurrence of density unevenness and stripe-shaped image defects in the formed image.
2. Discussion of Related Art
Conventionally, in the one-component non-magnetic development method, toner is supplied onto a developing roller, and the layer of toner on the developing roller is regulated with a regulating blade to a predetermined thickness to form a thin toner layer. Thereafter, a DC bias voltage or an AC bias voltage is applied to the developing roller either in or out of contact with a photosensitive member, thereby causing the toner to adhere to a latent image formed on the photosensitive member. Thus, the latent image is developed to form the desired image. In general, the developing roller in this type of image forming apparatus is driven through a gear train. Hence, the drive of the developing roller becomes unavoidably intermittent owing to the backlash in the mesh of the gears, causing feed irregularities to occur on the developing roller. Consequently, the thickness of a toner conveying surface formed on the developing roller becomes nonuniform in the circumferential direction. As a result, density unevenness (banding) occurs in the feed direction. Further, when the thickness of the toner layer on the surface of the developing roller is regulated with the regulating blade, toner is likely to clog in the nip between the developing roller and the regulating blade. For this reason, the thickness of the toner conveying surface formed on the developing roller may become uneven in the axial direction, resulting in an image containing longitudinal strip-shaped unevenness or longitudinal stripes. This causes the image quality to be degraded.
In contact development, the developing roller is brought into contact with the photosensitive member to perform development. In the contact development, toner may adhere to a non-image area, causing fogging. Further, blur may occur in a halftone image owing to disconnection, thickening or scattering of thin lines of the image. When printing is performed continuously, toner may adhere to the developing roller to give rise to a problem known as filming. This causes the toner transferability and chargeability to change, resulting in image quality degradation.
In non-magnetic color AC jumping development, an AC-superimposed bias voltage is applied to the developing roller to perform development in a state where the developing roller is out of contact with the photosensitive member. In the non-magnetic color AC jumping development, image force induced by small toner particles increases on the developing roller because of the increased charge of the small toner particles. For this reason, small toner particles cannot fly but remain in the developing device, resulting in an increase in the ratio of the amount of small toner particles remaining in the developing device to the amount of toner used for development. That is, selective consumption occurs. As a result, small toner particles are repeatedly stirred and pressed, causing filming on the developing roller and the regulating blade (i.e. the problem in terms of durability).
The magnetic brush development has the advantage that because the thickness of the toner layer can be increased, the development gap can be designed roughly. However, this development method involves the problem that it is costly because a magnet is installed on the developing sleeve, and the method cannot cope with the demand for color image formation. There are also problems inherent to this method. That is, owing to edge enhancement at the trailing end of the image, the density at the trailing end increases. Consequently, there may be undesired tailing produced by toner left uncollected by magnetic force (i.e. a phenomenon in which a trail of toner is undesirably left behind the image). With this method, the amount of toner conveyed is large, but on the other hand, charging of toner is likely to become insufficient, causing fogging.
Problems to be solved for the non-magnetic one-component development, exclusive of the magnetic brush development, include gradation degradation due to edge enhancement and an increase in density at the trailing end of the image. Regarding edge enhancement, some improvements can be made in general by reducing the development duty for adhesion of toner or applying an intermittent AC bias voltage to thereby shorten the period of time during which the toner adhesion bias voltage is applied. However, satisfactory improvements cannot be made by this method (i.e. the problem in terms of image quality, edge enhancement and fogging).
To reproduce halftone images, dithering and density pattern methods are used, and image processing using a screen structure is carried out. Various studies have been conducted and reported with regard to the relationship between the screen structure, e.g. the number of lines of the screen, and image quality, e.g. gradation reproduction and granularity. For example, it is reported in Japan Hardcopy ""99 Collection of Papers, pp. 303-306, Osamu Ide, et al. xe2x80x9cScreen Structure and Pixel Reproductionxe2x80x9d that the relationship between the dot gain, granularity and gradation reproduction was examined by measuring the pixel reproduction accuracy in images produced by electrophotography, printing and proofing with varied screen structures and various numbers of lines. According to the report, in systems of high accuracy, the image quality has a small dependence on the screen structure, but the dependence of the image quality on the screen structure increases as the accuracy lowers. Therefore, it is important to select an optimum screen in electrophotography and so forth.
Japan Hardcopy ""95 Collection of Papers, pp. 143-146, Yasuhiro Oda, et al. xe2x80x9cNew Screen Technique Designed with Importance Attached to Highlight Reproductionxe2x80x9d reports a technique in which the screen structure is varied according to the image density to realize stable reproduction of a highlighted region in a digital xerography system in which a halftone image is constructed by a halftone screen using an image modulation method.
There is an image forming apparatus in which an image is formed by developing an electrostatic latent image formed on a latent image carrier (photosensitive member) by using a contact developing device arranged to apply an AC-superimposed bias voltage to a developer carrier (developing roller) This type of image forming apparatus involves the problem that the resulting image is unfaithful to the latent image and inferior in gradation reproduction and halftone reproduction and hence suffers from poor image quality. To form a sharp color image, it is necessary to increase the amplitude (Vpp) of the AC-superimposed bias voltage applied to the developer carrier. However, the amplitude (Vpp) of the AC-superimposed bias voltage cannot sufficiently be increased because of such restrictions as the injection of an electric charge into the latent image carrier and background fogging. Accordingly, the conventional image forming apparatus of this type is not at a practical level.
These problems may be solved effectively by a method wherein the conveying surface of the developer carrier is formed in the shape of lines. However, this method involves the problem that noise may be generated depending on the screen structure or other factors. For example, if a halftone dot screen is used in a case where a line-shaped conveying surface is formed on the developer carrier, banding (lateral unevenness) occurs unfavorably. Further, if the frequency of the screen in the feed direction synchronizes to the frequency of a line-shaped conveying surface generated by application of an AC-superimposed bias voltage, density unevenness occurs on the latent image carrier. Alternatively, the superimposition of an alternating current on the developing bias voltage applied to the developer carrier causes the latent image carrier to be vibrated, which causes the nip to be varied. If the frequency of the screen in the feed direction synchronizes to the frequency of the nip variations due to the superimposition of the alternating current, density unevenness occurs on the latent image carrier.
Accordingly, an object of the present invention is to prevent the occurrence of banding and longitudinal unevenness and stripes and prevent fogging and scattering and further prevent the occurrence of filming, thereby allowing formation of a uniform image free from density unevenness.
To attain the above-described object, the present invention provides an image forming apparatus including a latent image carrier on which a latent image is formed. A developer carrier conveys a one-component non-magnetic developer and develops the latent image either in close proximity to or in contact with the latent image carrier. A voltage application unit applies an AC-superimposed bias voltage to the developer carrier. A regulating member abuts against the developer carrier to regulate the thickness of the developer layer on the developer carrier. The regulating member has a semiconductive member provided at the distal end thereof to produce an electric potential difference between the semiconductive member and the developer carrier, to which the AC-superimposed bias voltage is applied, when regulating the thickness of the developer layer on the developer carrier.
In addition, the present invention provides an image forming apparatus including a developing unit, a latent image carrier, and a latent image forming unit. A latent image formed on the latent image carrier by the latent image forming unit is developed by the developing unit to form an image. The developing unit applies an AC-superimposed bias voltage to a developer carrier to form a thin developer layer with a line-shaped uneven conveying surface on the developer carrier with a regulating member. The latent image forming unit forms a latent image on the latent image carrier by performing image processing using a vertical parallel-line screen.
Further, the latent image forming unit forms a latent image on the latent image carrier by performing image processing using a screen structure for reproducing a halftone image. The pitch of the line-shaped uneven conveying surface is set so as not to be an integral multiple nor quotient of the dot pitch in the feed direction of the screen. The frequency of the AC-superimposed bias voltage is set so as not to be an integral multiple nor quotient of the frequency of lateral unevenness occurring in the feed direction. Thus, the relationship between the pitch of the line-shaped uneven conveying surface produced on the developer carrier by the application of the AC-superimposed bias voltage and the pitch of stripes occurring in the feed direction is set so that the pitch of the line-shaped uneven conveying surface is not an integral multiple nor quotient of the pitch of stripes.
With the above-described structure, the present invention reduces the influence of the irregularity of feeding by the driving unit and minimizes the packing of the developer in the development nip, thereby increasing the degree of freedom with which the developer is movable, and faithfully reproducing the latent image to obtain a favorable image free from noise. In addition, the present invention minimizes fogging and scattering, reduces the occurrence of clogging with aggregates of developer and eliminates longitudinal strip-shaped unevenness and longitudinal stripes. Thus, it becomes possible to attain high image quality and to increase the lifetime of the system.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.